This invention relates to the field of navigation and is particularly suited for automatic ship steering.
In navigating a ship on the high seas it is desirable for reasons of speed, accuracy, efficiency and cost to hold the ship closely to a predetermined course or track. Because of the many dynamic disturbances a ship encounters at sea, such as currents, tides, winds and waves, as well as the absence of any recognizable visual cues, manual course corrections in the past were done only intermittently, allowing errors to build up. These course corrections primarily involved changes in heading from the present location toward the chosen destination and were based on compass indications and visual astronomical position fixes.
With the advent of mechanical motion measuring instruments and computers, automatic pilot systems were introduced to steer ships along a preset course. These systems were usually designed to hold a ship to a constant heading based on compass data inputs and required periodic manual corrections to compensate for accumulated track errors arising from environmental and systematic disturbances.
Precision course tracking is particularly important for survey ships that collect and map bathymetric, i.e. depth, gravimetric and geomagnetic data. These ships are required to collect and plot their data along prescribed courses, usually rhumb line tracks that run for long distances. A rhumb line track is a track which intersects all meridians at a constant angle. These surveys are usually carried out by traversing a region repeatedly along several closely spaced parallel rhumb lines and therefore require not only precise heading control but also precise positioning control. If the spacing between surveyed rhumb line tracks becomes too great due to the ship straying from the desired track, intermediate data may have to be collected to fill in gaps. If the tracks become too close or overlap, unnecessary redundancies may result. Because of the high cost of these surveys, it is essential that they be carried out accurately and efficiently. Thus there is a requirement for an autopilot steering system which will provide virtually instantaneous course corrections and continuous track control.